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National Advisory Committee for Aeronautics, Technical Notes - Propellant Vaporization as A Criterion for Rocket Engine Design; Calculations Using Various Log

Calculations were made to determine the vaporization rates of fuel
drops in a rocket engine for sprays having various log-probability dis-
tributions of asheptane drops. The rates were also calculated for various
engine design and operating parameters. The results indicate that a
longer chamber is required to obtain a given percent of fuel vaporized
for a spray with an increasing geometric standard deviation or an increas-
ing mass—median drop size. The calculations also indicate that a small
number of large drops that do not vaporize completely may be responsible
for the loss in engine performance. Experimental engine performance
results agree with the calculations for a spray having a geometric standard
deviation of 2.5 and mass-median drop radii of 70 to 280 microns, depending
on the type of injector. The calculations of the percent of fuel vaporized
can be correlated with an effective length for various engine design and
operating parameters.

Preliminary calculations (ref. 1) for the rate of fuel vaporization
in the combustion chamber of a heptane—oxygen rocket engine indicated
that propellant vaporization may be a rate-controlling step in the combus-
tion.process. The results of these calculations, based on a combustion
model in which vaporization of the fuel was rate-controlling, showed how
various design and operating parameter changes would effect the vaporiza-
tion rates of npheptane drops. The results were limited to one fuel
(heptane) and to the assumption that all of the fuel injected into the
chamber was of one drop size. It was pointed out that the experimental
results agreed with the calculated results except at the high percent of
fuel evaporated and percent theoretical performance levels. This differ-
ence in experimental and calculated results was explained qualitatively
as resulting from.a distribution of drop sizes.

The necessity for extending the calculations to consider a distribu— _
tion of drop sizes is evident, since such results would afford a more *
realistic correlation between the percent fuel evaporated and actual
rocket-engine performance values. This report covers the calculations
made at the NACA Lewis laboratory on the vaporization of various 103— E
probability distributions of fuel-drop sizes in a heptane-oxygen rocket 35
combustion chamber.